"It has to stop," ​said the president at a White House​ news conference​ on September 26.

​China has strongly denied involvement in a number of cyber crimes in recent years. Following the OPM hack in April, a spokesman for the Chinese embassy to the US called allegations “not responsible and counterproductive".

James R Clapper Jr, director of National Intelligence​, said​ the OPM case was not an ​"​attac​k" but instead​ a form of theft or espionage​, at a Senate hearing on September​ 29.

“We, too, practise cyber espionage and ... we’re not bad at it,” he said, cautioning the United States to tread carefully when punish​ing​ other countries​ for acts committed by its own intelligence services.

In November 2014, a​ ​cyber attack compromised the private files of more than 25,000 department of homeland security workers and thousands of other federal employees.